The goal of the proposed research is to identify and characterize the proteins involved in the replication of the genome of the DNA tumor virus SV40. We expect that much of what is learned about the replication of this simple viral chromosome will be relevant to understanding cellular DNA replication and its control. During the past grant period we achieved the reconstitution of origin-dependent DNA replication in vitro with purified proteins and characterized the biochemical mechanisms of initiation and elongation of nascent DNA chains. We also obtained cDNA clones corresponding to the cellular proteins involved in the initiation of SV40 DNA synthesis. During the next grant period we intend to make use of these reagents to analyze the functional roles of the individual cellular proteins in the initiation pathway and to characterize the critical interactions that determine the specificity and host range of viral DNA replication. We also plan to make use of the information obtained in these (and previous) studies to seek cellular counterparts of SV40 T antigen.